Fix limiting step re-execute bug (RIPD-1368):

The deferred credits table can compute a balance that's different from the
ledger balance.

Syntax:
A number written with no decimal means that number exactly. I.e. "12". A number
written with a decimal means that number has a non-zero digit at the lowest
order digit. I.e. "12.XX" means a number like "12.00000000000005"

Consider the following payment:
alice (USD) -> USD/XRP -> (XRP) Bob
Alice initially has 12.XX USD in her account.
The strand is used to debit alice the following amounts:
1) Debit alice 5
2) Debit alice 0.XX
3) Debit alice 3.XX

The next time the strand is explored, alice has a USD/XRP offer on the books,
and her account is credited:

1) Credit alice 20

When the beginning of the strand is reached, consider what happens when alice is
a limiting step. Calculate how much we can get out the step. According to the
deferred credit table this is:
12.XX - (5 + 0.XX + 3.XX)

This is also limited by alice's balance, which is large thanks to the credit she
received in the book step.

Now that the step has calculated how much we can get out, throw out the
sandbox (the one with the credit), and re-execute. However, the following error
occurs. We asked for 12.XX - (5 + 0.XX + 3.XX). However, the ledger has
calculated that alice has:
((12.XX - 5) - 0.XX) - 3.XX

That's a problem, because that number is smaller. Notice that there are two
precision losing operations in the deferred credits table:
1) The 5 + 0.XX step
2) The 12.XX - (total of debits). (Notice total of debits is < 10)

However, there is only one precision losing operation in the ledger calculation:
1) (Subtotal of 12.XX-5) - 0.XX

That means the calculation for the ledger results in a number that's smaller
than the deferred credits. Flow detects this as a re-execution error.

src/ripple/protocol/Feature.h

@@ -46,6 +46,7 @@ extern uint256 const featurePayChan;
 extern uint256 const featureFlow;
 extern uint256 const featureCryptoConditions;
 extern uint256 const featureTickSize;
+extern uint256 const featureRIPD1368;
 
 } // ripple